You can just use it at the end of your foe's turn and then untap and go on
While these variants are only available in Collector Boosters, players can get normal versions of these 17 cards by buying the Commander Deck they appear in
In decks with many creatures you may want to run at least 1 board protection card with more ( 2-3 ) to be recommended unless you can have other ways to recover your board like mass recursion
But when should you play a card like this
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Hedron Crab is certainly the more flexible of the two because you can use it to enable your own graveyard, but both of these cards are fantastic mill engines for 60-card formats